Laser with tunable reflector

Abstract

An optoelectronic device includes a gain medium having a first end and a second end and configured to amplify laser radiation within a gain band having a peak at a given wavelength. A laser cavity containing a gain medium includes a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite the first side. The second reflector has a reflectance as a function of wavelength that is adjustable so as to reduce the reflectance of the second reflector at the peak of the gain band, thereby broadening the spectrum of laser radiation emitted by the second reflector from the gain medium.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 538,487, filed on September 14, 2023, which is incorporated herein by reference. Technical Field

[0003] The present invention relates generally to optoelectronic devices and, more particularly, to semiconductor lasers. Background Art

[0004] Wavelength domain multiplexing (WDM) is used in high-speed optical communications to enable multiple communication signals with different carrier wavelengths to be transmitted on the same optical fiber. Some WDM devices use a single laser that emits radiation in a comb of wavelengths to provide carriers of multiple wavelengths simultaneously. The term "comb" as used in this specification and claims refers to a set of different spectral lines that are spaced apart by equal steps in wavelength. The term "comb filter" refers to an optical filter having a passband composed of such a comb, so that the comb filter passes light having wavelengths in a set of different spectral lines while blocking wavelengths between the lines in the set. The terms "light" and "optical radiation" are used synonymously in this specification and claims to refer to any electromagnetic radiation in the infrared, visible and ultraviolet spectral ranges.

[0005] Mach-Zehnder interferometers (MZIs) are used in various silicon photonics applications. For example, El Shamy et al. describe MZI-based tunable filters in "Modelling, characterization, and applications of silicon on insulator loopterminated asymmetric Mach Zehnder interferometer", Scientific Reports 12: 3598 (2022). The authors describe a loop-terminated MZI whose spectrum can be tuned not only by controlling the phase difference between the arms of the interferometer, but also by using its directional coupler coefficients to form a spectrally tunable filter. Summary of the invention

[0006] Embodiments of the present invention described below provide semiconductor laser devices that output optical radiation within a range of wavelengths, and methods for producing and operating such devices.

[0007] Therefore, according to an embodiment of the present invention, an optoelectronic device is provided, which includes a gain medium having a first end and a second end and is configured to amplify laser radiation within a gain band having a peak at a given wavelength. The laser cavity contains the gain medium and includes a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite to the first side. The second reflector has a reflectivity as a function of wavelength, and the reflectivity is tunable so as to reduce the reflectivity of the second reflector at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

[0008] In a disclosed embodiment, the apparatus includes a bandpass filter disposed within the laser cavity, the bandpass filter having a passband encompassing a peak of the gain band.

[0009] Additionally or alternatively, the apparatus includes a comb filter disposed within the laser cavity and configured to pass a set of different wavelength sub-bands within the gain band, the set of different wavelength sub-bands defining a comb, wherein tuning the second reflector to reduce reflectivity increases the number of wavelength sub-bands in the laser radiation emitted from the gain medium. In a disclosed embodiment, the comb filter includes an optical ring resonator.

[0010] In some embodiments, the second reflector comprises a tunable Mach-Zehnder interferometer (MZI).In disclosed embodiments, the tunable MZI comprises a pair of waveguides disposed on a substrate and a heater disposed on the substrate and coupled to change a temperature of at least one of the waveguides.

[0011] Additionally or alternatively, the gain medium and the first reflector are included together in a reflective semiconductor optical amplifier (RSOA), and the second reflector is included in an external laser cavity disposed on the optical substrate and optically coupled to the RSOA. In one embodiment, the RSOA includes a III-V semiconductor material and the external laser cavity includes a silicon photonic integrated circuit (PIC).

[0012] According to an embodiment of the present invention, a method for generating radiation is also provided. The method includes providing a gain medium having a first end and a second end and configured to amplify laser radiation within a gain band having a peak at a given wavelength. The gain medium is inserted into a laser cavity, the laser cavity including a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite to the first side, the second reflector having a reflectivity that is tunable as a function of wavelength. The second reflector is tuned to reduce the reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be more fully understood from the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which:

[0014] Figure 1 is a block diagram schematically showing a multi-wavelength laser device according to an embodiment of the present invention;

[0015] Figure 2A According to an embodiment of the present invention Figure 1 Schematic detail of a tunable reflector used in the device;

[0016] Figure 2B yes Figure 2A Schematic diagram of the reflectivity of a tunable reflector;

[0017] Figure 3 According to an embodiment of the present invention Figure 1 Schematic diagram of the spectral characteristics of the laser device;

[0018] Figure 4 and Figure 5 is a schematic diagram showing an embodiment of the present invention. Figure 1 a graph showing the spectral characteristics of a laser device tuned by

[0019] Figure 6 According to an embodiment of the present invention Figure 1 A graph of the output spectrum of a laser device. DETAILED DESCRIPTION

[0020] Some multi-wavelength semiconductor lasers used in WDM applications emit a comb of wavelength sub-bands extending over a specific spectral range. The density of the comb (i.e., the spacing between the center wavelengths of the sub-bands) is determined by the laser cavity length, while the spectral range is primarily limited by the width of the gain band of the optical gain medium in the laser. The gain band typically has a peak at a certain wavelength and falls off sharply on either side of the peak. The gain medium is contained in a laser cavity that includes reflectors on opposite sides of the gain medium and a comb filter inside or outside the laser cavity.

[0021] In order to increase the information carrying capacity of a WDM communication link, it is desirable that the laser driving the link emits a broad comb of many wavelengths with approximately equal output power in all wavelengths of the comb. To this end, in an embodiment of the present invention, the output reflector of the laser cavity containing the gain medium has a tunable spectral reflectivity (i.e., a reflectivity that varies as a function of wavelength). The output reflector is tuned to reduce its reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

[0022] The broadened spectrum enables the laser device to output a comb that includes a large number of wavelength sub-bands for a given spacing between sub-bands. Alternatively, the principles of the present invention can be applied to broaden the spectral output band of other types of lasers, not just comb lasers. In either case, tuning of the spectral reflectivity of the output reflector produces a broad, flat laser output spectrum without compromising the efficiency of the laser, because the power output capability of the gain medium is fully utilized, i.e., increased losses at the peak of the gain band are converted into increased laser power output in the wings of the gain band.

[0023] In some embodiments, the output reflector comprises a tunable Mach-Zehnder interferometer (MZI) comprising a pair of waveguides disposed on a substrate such as a photonic integrated circuit (PIC). A heater on the substrate is driven to change the temperature of one or both waveguides, thereby adjusting their relative optical lengths. The resulting phase difference of the radiation propagating in the two waveguides determines the fraction of energy output by the reflector as a function of wavelength. The spectral reflectivity is tuned by controlling the temperature of the heater. Alternatively, other types of broadband tunable optical elements may be used to control the spectral reflectivity of the output reflector.

[0024] Figure 1 is a block diagram schematically illustrating a multi-wavelength laser device 20 according to an embodiment of the present invention. The device 20 includes an active optical chip 22 on a III-V semiconductor substrate 26 such as GaAs or InP, and a PIC 24 on an optical substrate 40 such as a silicon-on-insulator (SOI) substrate. For mechanical stability and compact packaging, the chip 22 can be bonded to the optical substrate 40, for example using a suitable adhesive. Alternatively, the chip 22 and the PIC 24 can be mounted separately and connected, for example, by a suitable optical fiber.

[0025] The chip 22 includes a reflective semiconductor optical amplifier (RSOA) 28, which includes an optical gain medium 30 that amplifies laser radiation within a specific gain band, and a reflector 36 formed at the inner end of the gain medium 30. Application of a drive voltage between electrodes 32 and 34 on the chip 22 results in optical gain in the medium 30 across the gain band (depending on the type of gain medium used, this is, for example, over a band of tens of nanometers centered around a peak at about 1310 nm or 1550 nm). The gain medium 30 may include, for example, GaInAsP, AlGaInAs quantum wells, or InAs quantum dots that emit light in these gain bands. The reflector 36 may include, for example, a distributed Bragg reflector (DBR), or a reflective coating on the back facet of the RSOA. A waveguide 38 on the substrate 26 transmits the laser radiation into and out of the RSOA at the end opposite to the reflector 36.

[0026] PIC 24 includes an external laser cavity 42, which is optically coupled to waveguide 38 and terminated in a tunable reflector 50. Reflector 50 is partially reflective to allow the multi-wavelength laser beam generated by device 20 to exit the cavity and has a tunable reflector as a function of wavelength. Specifically, in this embodiment, reflector 50 is tuned to reduce its reflectivity at the peak of the gain band of gain medium 30. This function of reflector 50 will be described in more detail below.

[0027] The cavity 42 includes a comb filter 44 and a bandpass filter 46 connected in series, and a phase tuner 48 for adjusting the cavity phase. In this example, the comb filter 44 includes an optical ring resonator whose ring length is selected so that the filter 44 passes a comb of wavelength sub-bands within the gain band of the RSOA 28. Thus, the multi-wavelength laser device 20 can be used as a beam source for WDM communication applications, where, for example, the wavelength spacing between the sub-bands in the comb is set equal to the spacing between the WDM channels. The bandpass filter 46 has a passband that includes the peak of the gain band of the gain medium 30. The structure and operation of the comb filter 44 and the bandpass filter 46 can be based on, for example, the components of the comb laser described in U.S. patent application 18 / 366,705 filed on August 8, 2023, the disclosure of which is incorporated herein by reference. Alternatively, a tunable output reflector of the type described herein can be used to broaden the spectral range of other types of semiconductor lasers and cavity designs known in the art.

[0028] Figure 2A is a schematic detail view of a tunable reflector 50 according to an embodiment of the present invention. In this embodiment, the reflector 50 comprises a thermally tunable MZI comprising a pair of waveguides 52, 54 on a substrate 40, and corresponding heaters 62, 64 comprising resistive traces on the substrate 40. The waveguides 52 and 54 are coupled together at their ends by directional couplers 56 and 58 and a terminal loop 60 at the inner end of the MZI. The coupling coefficients of the directional couplers 56 and 58 determine the relative amount of laser energy split between the waveguides 52 and 54 and thus set the basic reflectivity of the reflector 50 (e.g., as described in the above-mentioned article by El Shamy et al.). At the outer end of the MZI, the waveguide 52 is coupled to the cavity 42, while the waveguide 54 outputs a laser beam from the cavity.

[0029] To control the spectral reflectivity of the reflector 50, different currents are applied to the heaters 62 and 64 (or current is applied to only one heater) through the electrical terminals 66 on the substrate 40. The temperature difference between the waveguides 52 and 54 results in an effective optical path difference of the waveguides, which in turn tunes the phase difference between the light waves that meet at the directional couplers 56 and 58. This phase difference, which varies as a function of wavelength, controls the spectral reflectivity of the reflector 50, i.e., the ratio between the energy output from the waveguide 54 and the energy reflected back to the cavity 42 via the waveguide 52 at each wavelength within the gain band of the laser device 20. This spectral reflectivity can be tuned in amplitude and wavelength by varying the current applied to the heaters 62 and / or 64.

[0030] Figure 2B is a diagram showing reflectivity 70 of tunable reflector 50 as a function of wavelength according to an embodiment of the present invention. In this example, the currents applied to heaters 62 and 64 are selected to minimize the reflectivity at about 1320 nm near the peak of the gain band of gain medium 30. The reflectivity is about 0.05 at the gain band peak and increases over a range of 50 nm on either side of the peak.

[0031] Figure 3 80 is a graph schematically showing the spectral characteristics of the laser device 20 according to an embodiment of the present invention. －1 ) shows the gain of the gain medium 30 as a function of wavelength (λ) in arbitrary units. Curve 82 shows the net loss of the laser cavity as a function of wavelength in the same units as the gain. The net loss includes a fixed loss that is generally approximately constant as a function of wavelength and the transmittance (inverse of the reflectance) of the reflector 50 that varies as a function of wavelength, as shown in FIG. Figure 2B shown.

[0032] The condition for lasing at a given wavelength is that gain (λ) ≈ net loss (λ), which means that the net gain (the difference between the gain and the net loss) is approximately zero. Curve 84 shows the net gain as a function of wavelength. Due to the reduced reflectivity of reflector 50 (shown by curve 82) at the peak of the gain band (shown by curve 80), the net gain has a value of 0 cm over a range extending from about 1300 to 1320 nm. －1 A wide flat peak.

[0033] Figure 4 and Figure 5 are graphs schematically illustrating the tuning of the spectral characteristics of the laser device 20 according to an embodiment of the present invention. In both graphs, curve 86 shows the additional loss due to the bandpass filter 46, which is tuned to select the operating wavelength range of the laser device. Figure 4In FIG. 1 , the bandpass filter 46 and the reflector 50 are tuned to shift the output wavelength range of the laser device 20 to shorter wavelengths within a range centered around 1309 nm; and Figure 5 The output wavelength range is centered around 1322 nm.

[0034] Figure 6 is a graph schematically showing an output spectrum of the laser device 20 according to an embodiment of the present invention. Figure 3 The curves shown, curve 84 shows the net gain of the laser as a function of wavelength, and curve 94 shows the net gain of the comb filter 44 ( Figure 1 ). Curve 92 shows the resulting output spectrum of the laser device, resulting in a broad comb with different wavelength sub-bands 94 of approximately equal output power levels and approximately 1 nm wavelength spacing. As previously described, the range of comb wavelengths can be shifted by appropriate tuning of the bandpass filter 46 and reflector 50. Alternatively, the output characteristics of the laser device, such as the width of the emission band and the number of sub-bands in the comb, can be varied by appropriate changes to the laser cavity parameters.

[0035] The above embodiments are cited by way of example, and the present invention is not limited to what has been specifically shown and described above. On the contrary, the scope of the present invention includes the combination and sub-combination of the various features described above, as well as changes and modifications thereof that will occur to those skilled in the art after reading the foregoing description and are not disclosed in the prior art.

Claims

1. An optoelectronic device, comprising: a gain medium having a first end and a second end and configured to amplify laser radiation within a gain band having a peak at a given wavelength; as well as A laser cavity containing the gain medium and comprising: a first reflector disposed on a first side of the gain medium; as well as a second reflector disposed on a second side of the gain medium opposite to the first side, the second reflector having a reflectivity as a function of wavelength, the reflectivity being tunable so as to reduce the reflectivity of the second reflector at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

2. The device according to claim 1, comprising: A bandpass filter is disposed in the laser cavity and has a passband surrounding the peak of the gain band.

3. The device according to claim 1, comprising: a comb filter disposed within the laser cavity and configured to pass a set of different wavelength sub-bands within the gain band, the set of different wavelength sub-bands defining a comb, wherein tuning the second reflector to reduce the reflectivity increases the number of the wavelength sub-bands in the laser radiation emitted from the gain medium. The apparatus of claim 3 , wherein the comb filter comprises an optical ring resonator.

5. The apparatus of claim 1, wherein the second reflector comprises a tunable Mach-Zehnder interferometer (MZI).

6. The apparatus of claim 5, wherein the tunable MZI comprises: a pair of waveguides disposed on a substrate; and a heater disposed on the substrate and coupled to change a temperature of at least one of the waveguides.

7. The apparatus of claim 1 , wherein the gain medium and the first reflector are together included in a reflective semiconductor optical amplifier (RSOA), and the second reflector is included in an external laser cavity disposed on an optical substrate and optically coupled to the RSOA.

8. The apparatus of claim 7, wherein the RSOA comprises a III-V semiconductor material, and wherein the external laser cavity comprises a silicon photonic integrated circuit (PIC).

9. A method for generating radiation, the method comprising: providing a gain medium having a first end and a second end and configured to amplify laser radiation within a gain band having a peak at a given wavelength; inserting the gain medium into a laser cavity, the laser cavity comprising a first reflector disposed on a first side of the gain medium and a second reflector disposed on a second side of the gain medium opposite the first side, the second reflector having a tunable reflectivity as a function of wavelength; as well as The second reflector is tuned to reduce the reflectivity at the peak of the gain band, thereby broadening the spectrum of the laser radiation emitted from the gain medium through the second reflector.

10. The method of claim 9, comprising inserting a bandpass filter in the laser cavity, the bandpass filter having a passband including the peak of the gain band.

11. The method of claim 9, comprising inserting a comb filter within the laser cavity, the comb filter configured to pass a set of different wavelength sub-bands within the gain band, the set of different wavelength sub-bands defining a comb, wherein tuning the second reflector to reduce the reflectivity increases the number of the wavelength sub-bands in the laser radiation emitted from the gain medium.

12. The method of claim 11, wherein inserting the comb filter comprises coupling an optical ring resonator within the cavity.

13. The method of claim 9, wherein inserting the gain medium comprises coupling a tunable Mach-Zehnder interferometer (MZI) to serve as the second reflector.

14. The method of claim 13, wherein coupling the tunable MZI comprises providing a pair of waveguides and a heater on a substrate, and wherein tuning the second reflector comprises controlling the heater to change a temperature of at least one of the waveguides.

15. The method of claim 9, wherein inserting the gain medium comprises providing a reflective semiconductor optical amplifier (RSOA) comprising the gain medium and the first reflector, and optically coupling an external laser cavity to the RSOA, wherein the external laser cavity is disposed on an optical substrate and comprises the second reflector.

16. The method of claim 15, wherein providing the RSOA comprises forming the RSOA on a III-V semiconductor material, and wherein optically coupling the external laser cavity to the RSOA comprises: The RSOA is coupled to a silicon photonic integrated circuit (PIC) via a waveguide.

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